Refining of pig iron

ABSTRACT

It has been found that the carbon monoxide in gases emerging from a converter tends to burn to carbon dioxide by contact with oxygen in the surrounding air and that the flame so produced has an intensity which is closely related to the rate of flow of carbon monoxide, and hence to the rate of decarbonization of the iron. This is made use of in the invention by controlling the flow of gases emerging from the converter spout and measuring, generally continuously, the intensity of the flame produced when placed in contact with the surrounding air in a hood.

United States Patent Denis 51 Mar. 28, 1972 54] REFINING 0F PIG IRON 2,354,400 7/1944 Percy ..75/60 2,803,987 8/ i957 Galey ..75/60 UK [72] Invent Belgmm 2,807,537 9/1957 Murphy ..75/60 [73] Assignee: Centre National De Recherches Metallur- 3,222,045 12/1965 Spetzler ..7$/60 X glques, Brussels, Belgium Primary Examinerl..' Dewayne Rutledge [221 1967 Assistant Examiner-G. K. White [21] App]. No.: 661,401 Attorney-Holman8t8tern [30] Foreign Application Priority Data [57] ABS CT It has been found that the carbon monoxide in gases emerging g 3g tuxemgomg from a converter tends to burn to carbon dioxide by contact 0mg with oxygen in the surrounding air and that the flame so produced has an intensity which is closely related to the rate E (El of flow of carbon monoxide, and hence to the rate of damp 58] d 75/60 bonization of the iron. This is made use of in the invention by re o are controlling the flow of gases emerging from the Convener spout and measuring, generally continuously, the intensity of [56] References Cited the flame produced when placed in contact with the surround- UNITED STATES PATENTS mg air In a hood- 4 Claims, 2 Drawing Figures 2,207,309 7/1940 Work ..75/6O PAIENTEIJMARzsmz V 3,652,262

SHEET 1 [1F 2 SHEET 2 BF 2- l|: ||||1 [1 --CALCULATED SPEED OF DECARBONISATION Kq min INTENSITY OF THE FLAME A V Vxm Y 400 I/A/ W fiv x ZOO x REFINING OF PIG IRON The present invention relates to a method for controlling the refining operation of pig iron carried out in a converter.

It is known that during the whole of the refining operation of pig iron the oxidizing gas injected into the liquid metal in the converter has the effect of oxidizing the undesirable elements contained in the said metal, i.e., phosphorus, silicon, manganese, and sulphur, as well as the greater part of the carbon. The first four elements mentioned are found in oxidized form in the slag, whereas the carbon, which burns to form CO and then possibly to form CO escapes from the converter based on the extent to which the decarbonization of the pig iron progresses.

The rate of flow of CO and CO escaping from the converter can constitute a means for measuring the state of progress of the phenomenon of decarbonization, since during decarbonization the carbon contained in the CO and in the CO comes practically exclusively from the metal in the converter.

A number of methods have already been proposed for measuring the progress of the decarbonization of the pig iron by means of measuring the rates of flow of gas issuing from the converter. A number of these methods are based on measuring the amount of heat supplied by the practically complete combustion of the CO into CO in the interior of a suction hood for the gases issuing from the converter. The said processes have in general the disadvantage that the indications they supply are not very accurate and are frequently delayed.

The present invention aims at producing a method by which it is possible easily to observe the evolution of the phenomenon of decarbonization of the pig iron, while avoiding the disadvantages mentioned above.

The method of the invention, in which pig iron in a converter is subjected to a top refining operation, has the essential feature that the gases issuing from the converter are drawn through a hood arranged above the converter, the reduced pressure creating vacuum effect proportional to the rate of flow of the gases issuing from the converter, in such a way that during the refining operation the surrounding air cannot be drawn into the hood with the said gases. The intensity of the radiation of the flame is measured in the course of its passage between the spout of the converter and the hood, which makes it possible to deduce the rate of decarbonization of the pig iron.

It has been ascertained that the CO issuing from the converter burns by the oxygen of the surrounding air and produces a radiation whose intensity varies very substantially with the total rate of flow of the CO. As the rate CO/CO of the gases issuing from the converter remains practically constant during the major part of the refining operation, it follows that the intensity of the radiation observed is in direct relation to the total amount of C which escapes at that moment from the converter and consequently to the rate of decarbonization of the pig iron. Therefore the rate of flow through the hood is in proportion to the rate of issuance of CO from the converter, since no re-entry of air into the hood is possible, the intensity of this phenomenon of peripheral combustion, and consequently the intensity of the radiation resulting from this combustion, makes it possible to follow particularly accurately and rapidly the progress of the decarbonization of the pig iron. Accordingly one has an effective means to control of the refining operation ofthe pig iron.

in accordance with an advantageous variation of the method of the invention, the measurement takes place continuously, for instance by means of a cell which is sensitive to infra-red radiations and which is connected to a recording member.

The operator supervising the graph as it develops, produced by the recording member, is accordingly given almost instantaneous information on the state of progress of the decarbonization, and can take opportunely any measure or countermeasure which may be judged to be useful or necessary.

The invention will be better understood by reference to the accompanying drawings in which:

FIG. 1 schematically shows the entire system used, from the converter to the recording instruments;

FIG. 2 shows the course of the two recorded curves, i.e., the speed of decarbonization of the metal bath and the intensity of the flame arising from the combustion of CO in the hood as a function of time.

FIG. 1 shows an elevation of a basic oxygen furnace (converter) containing the melt subjected to the refining operation at the top by means of an oxygen jet 2. Also shown is the catchment hood 3 for the fumes leaving the converter 1.

The flame which follows the gases leaving the converter 1 and passing through the hood 3 consists of a central spear (4), which is substantially conical and contains chiefly CO, and unburned CO which has not been mixed with air. This spear (4) is capped by a brighter gas envelope (5) or zone which is the area of combustion of CO by the air.

According to the invention the gases leaving the converter 1 are sucked into the hood and the intensity of the radiation emitted by the zone (5) where the combustion of CO by the oxygen of the air takes place is measured.

The cell 6 viewing the zone 5 of the flame is placed at such a point in the hood 3 that its line of sight 7 meets the zone (5) at a greater distance and the spear (4) at a lesser distance, preference being given to a line of sight whose axis 7 is close to the point of the spear (4) without touching it, as shown in FIG. 1.

The viewing cell 6 is connected to a recording instrument 9 by means of an appropriate circuit 8, comprising means of amplification, adaptation and thermal compensation of the transmitted signals. The recording instrument 9 yields the graph II shown in FIG. 2, this graph representating as a function of time (on abscissa) the measurement of the intensity of the flame (on ordinate, conventional units) due to the combustion of CO in the hood.

The hood 3 is also provided with a gas pickup 10 connected to an analyzer 11 adapted to indicate the concentrations of CO and CO in the combustion (burned) gases. Another gas pickup 12, on the other hand, measures the gas output by means of an output meter 13. The data collected by the analyzer 1 l and the output meter 13 are then fed into an analogue computer 14, whose output signal is passed on to a recorder 15 which yields the curve of the rate of decarbonization of the metal bath contained in the converter 1. This curve of the rate of decarbonization is the curve 1 shown in FIG. 2, where the ordinate is kilogrammes of carbon per minute and the abscissa is in minutes.

A close analogy will be observed between the curve given by the recorder 9 and that given by the recorder 15, whereby to any variation in the one there corresponds a proportional variation of the same sense in the other.

Seeing that the curve traced by the recorder 9 is a graph derived from measurements, it is obtained almost instantaneously and by means of relatively simple apparatus. On the other hand, the curve yielded by the recorder 15 is a graph arising from calculations the results of which are not known till after a certain interval by means of such apparatus as, for instance, an analogue computer. In general the curve supplied by the recorder 15 cannot be obtained till after a minute to a minute and a half after that supplied by the recorder 9.

These considerations emphasize the importance of the present invention, which lies in the fact of the calculated curve yielded by the recorder 15 being replaced by the measurement curve traced by the recorder 9. The advantages of the measurement graph lie essentially in the greater simplicity of the means employed, which is also reflected in a reduction of cost and a shortening of the response interval.

The curves I and II in FIG. 2 apply to the case where a sufficient amount of air is drawn into the hood to ensure substantially complete combustion of the CO contained in the gases leaving the converter.

The curve I shows on the ordinate, in kilogrammes of carbon per minute, the speed of decarbonization of the metal bath calculated as a function of time plotted on the abscissa the two graphs are striking and the importance of replacing the calculated curve (I) by the measured curve (ll) will be readily appreciated.

The curve l shows the speed of decarbonization during the first refining stage of a phosphorous melt by means of oxygen blown over its top.

During the first 2 minutes of refining the speed of decarbonization rises rapidly; then lime is injected into the converter, which together with the slag formation at first causes a drop in the speed of decarbonization, which touches a minimum between the fifth and the sixth minute from the beginning of the refining operation. Once slag has been formed in sufficient amount, the speed of decarbonization begins to rise again approximately up to the 11th minute, which corresponds to the terminal period of the first refining phase and is characterized by a continued steep drop in the speed of decarbonization.

l claim:

1. In a method of controlling the refining of pig iron in a converter by top blowing oxygen through the molten pig iron in the converter, allowing the gases to emerge from the spout thereof, positioning a hood over said spout, and drawing the gases emerging from the spout through said hood, whereby a flame is fonned having a central spear capped by a combustion zone wherein C is burned to C0,, the improvement comprising measuring the intensity of radiation emitted by said combustion zone along a line of sight passing through the combustion zone ahead of said central spear, the intensity measured being in direct relation with the rate of decarbonization of said molten pig iron, so as to instantaneously be able to accurately determine the amount of carbon removed from said molten pig iron at any given point in time.

2. A method as claimed in claim 1, in which the volume of air inside said hood is increased by drawing a known quantity of external air through the hood so as to burn to CO, the CO in said gases emerging from the spout of the converter.

3. A method as claimed in claim 2 inwhich the air inside the hood, including the external air drawn in, is sufficient to burn to CO essentially all the CO in said gases issuing from the spout of the converter.

4. A method as claimed in claim 1 in which said measuring the intensity of the flame step is carried out continuously during the refining operation and the measurements are continuously recorded. 

2. A method as claimed in claim 1, in which the volume of air inside said hood is increased by drawing a known quantity of external air through the hood so as to burn to CO2 the CO in said gases emerging from the spout of the converter.
 3. A method as claimed in claim 2 in which the air inside the hood, including the external air drawn in, is sufficienT to burn to CO2 essentially all the CO in said gases issuing from the spout of the converter.
 4. A method as claimed in claim 1 in which said measuring the intensity of the flame step is carried out continuously during the refining operation and the measurements are continuously recorded. 